1. Field of the Invention
The present invention relates to an apparatus and a method for the optoelectronic classification and separation of semiconductor material.
2. The Prior Art
High-purity semiconductor material is required for the production of solar cells or electronic components, such as for example storage elements or microprocessors. The dopants introduced in a targeted manner are the only impurities which, in the most favorable case, a material of this kind should contain. It is therefore desirable to keep the concentration of harmful impurities as low as possible. It is frequently observed that even semiconductor material which has been produced to a high level of purity is recontaminated during further processing to give the desired products. For this reason, complex cleaning steps are required again and again in order to regain the original level of purity. Atoms of foreign metals which become incorporated into the crystal lattice of the semiconductor material can interfere with the charge distribution. These atoms can reduce the performance of the ultimate component or lead to its failure. Consequently, contamination of the semiconductor material resulting in particular from metallic impurities is to be avoided. This applies in particular to silicon, which is the most frequently used semiconductor material in the electronics industry. High-purity silicon is obtained, for example, by the thermal decomposition of silicon compounds which are highly volatile, and are therefore easy to purify using a distillation method, such as for example trichlorosilane. In this case, the silicon is obtained in the form of polycrystalline rods with typical diameters of from 70 to 300 mm and lengths of from 500 to 2500 mm. A large proportion of the rods are used to produce crucible-pulled monocrystals, strips or sheets, or to produce polycrystalline solar-cell base material. Since these products are made from high-purity, molten silicon, it is necessary to melt solid silicon in crucibles. In order for this operation to be as efficient as possible, large-volume, solid silicon pieces, such as for example the abovementioned polycrystalline rods, have to be comminuted prior to melting. This generally entails surface contamination of the semiconductor material, since the comminution is carried out using metallic crushing tools, such as jaw or rolling crushers, hammers or chisels.
According to the usual comminution methods for semiconductor materials using mechanical tools, such as crushers or hammers, the semiconductor material is present in various fragment sizes. For process engineering reasons, numerous semiconductor materials, such as primarily polysilicon, have to be present in a specific fragment size distribution for the melting operation. Since it is not permissible for any impurities to pass into the crucible together with the semiconductor material, very particular demands have to be placed on both the crushing process and on the classification process, so that there is no contamination from atoms of foreign material emanating from metallic tools, such as for example screening apparatus. This fact precludes conventional screening apparatus which are commercially available. When screening on, for example, a vibrating screen made of metal, the hard, sharp-edged silicon fragment leads to a high level of abrasion of the screen bottom and therefore to unacceptable contamination of the silicon surface, requiring the use of complex purifying methods. Therefore, screen bottoms made of silicon are used. However, the high risk of the silicon components breaking entails a high outlay on refitting. A further drawback of screening methods is the high risk of the screen becoming blocked, due to the irregular grain shape of the silicon fragments.
For these reasons, the use of screen-free separating methods, such as fluid separation and classification, was investigated. Since the required cut-off points lie in the range of centimeters, gas-separation and classification is ruled out. This is because the high air velocities required for this purpose, combined with the sharp-edged material to be screened, cause a high level of abrasion to the equipment. Fluid separation in water exhibits this drawback only to a limited extent. However, in this case the irregular grain shape of the silicon fragment leads to a very imprecise cut-off point. This is because, for example, leaf-shaped silicon fragments are suspended in the fine material due to their low sinking rate, even though their geometric dimensions mean that they belong to a coarser grain class. Moreover, in this wet classification and separation method, continuous delivery of material is very difficult.
Thus all the classification and separation methods which have been described above exhibit significant drawbacks, since they either contaminate the material to be screened, tend to cause a blockage or have an insufficiently accurate cut-off point.